Network and Subnetwork variables' default values

Currently network and subnetwork variables' default values are null. This should be

  network    = "projects/${local.project}/global/networks/default"
  subnetwork = "projects/${local.project}/regions/${local.region}/subnetworks/default"

to avoid Terraform wanting to re-create the cluster every single time.

This is not causing any problem in Moodle Cloud because it sets those vars in the locals.tf file.

As we are not customising these parameters anywhere, I propose to remove them to simplify the module.